Maybe you'll notice that you actually prefer guilt and frustration to the pride, accomplishment, and pleasure that come with doing what you set out to do. Crazy as it sounds, lack of motivation can be related to secretly wanting to feel and think negatively about yourself. You may prefer these feelings, as unpleasant as they are, to the anxiety that comes with making big changes and opening yourself up to new possibilities in your life. If this rings a bell for you, you may need to work on getting comfortable with positive thoughts and feelings about yourself.
Either way, paying attention to what's going on inside when you act is the first step towards uncovering your own intrinsic motivation to accomplish your goals. If you can commit to keeping your Before-During-After journal faithfully—even for a short time, like two or three weeks—I’ll bet big bucks that your problems with feeling unmotivated will happen much less often and be much easier to deal with.